1. Field of the Invention
The present invention relates to a signal processing circuit for a rotation detecting device. In particular, the present invention relates to a signal processing circuit for a rotation detecting device using non-contact type magnetic sensors. More particularly, the invention includes a signal processing circuit that, when a plurality of magnetic sensors, disposed opposed to a rotating body on which gear teeth are formed, outputs rectangular first and second sensing signals both having a rectangular waveform but differing in phase, and outputs a detection signal for detecting a rotational position of the rotating body based on the first and second sensing signals.
2. Description of the Related Art
A technique described in, for example, Japanese Patent Laid-open Publication No. 2007-170922 is known as a conventional signal processing circuit for a rotation detecting device. The rotation detecting device described in this Publication No. 2007-170922 includes two magnetic sensors that output rotation detection signals Sa and Sb having differing phases when a rotor on which gear teeth are farmed rotates. The signal processing circuit judges that a rotation direction of the rotor has been reversed based on a change in the relative phase of the rotation detection signals Sa and Sb, such as that as shown in FIG. 1, and generates a reverse signal. The signal processing circuit also detects the edges of the rotation detection signal Sa.
The signal processing circuit then generates a level-switching inactivation signal that prohibits signal level switching performed synchronously with both the first rising edge and the first failing edge of the signal Sa after reversed rotation of the rotor is detected. Based on the level-switching inactivation signal, the signal processing circuit generates a first output signal OUT1 by masking the signal Sa for the duration of a single pulse immediately after the reversed rotation of the rotor is detected, and outputs the generated first output signal OUT1. Moreover, based on the first output signal OUT1 and the reverse signal, the signal processing circuit generates a second output signal OUT2 and outputs the generated second output signal OUT2. The second output signal OUT2 changes between a high level and a low level, when the rotor is rotating in a normal direction, and changes between a high level and a middle level, when the rotor is rotating in the reverse direction.
However, when the signal Sa is simply masked for the duration of a single pulse immediately after the reversed rotation of the rotor is determined as in the above-described conventional technology, an accurate rotational position of the rotor is difficult to detect from the second output signal that is ultimately generated and outputted. The reasons for this difficulty will be described with reference to FIG. 1.
As shown in FIG. 1, when the rotor is rotating in the normal direction, the rising edges of the second output signal OUT2 correspond to the positions of the front edges in the rotation direction of a tooth A, a tooth B, and a tooth C on the rotor. Therefore, the positions of the rotational front edge of each tooth can be detected based on the rising edge of the second output signal OUT2.
However, when a tooth on the rotor is detected by the rising edge of the second output signal OUT2 in the same manner and the rotation direction of the rotor changes from normal to reverse, a discrepancy occurs in the detection result. As shown in FIG. 1, the detection result is tooth B when, in actuality, the tooth is tooth A.
To prevent such discrepancies in the detection results, detecting the position of each tooth based on the falling edge of the second output signal OUT2 can be considered when the rotor is rotating in the reverse direction. However, in this instance, the edge of the second output signal OUT2 used for detecting the teeth on the rotor is required to be switched between the rising edge and the falling edge based on the rotation direction of the rotor. Therefore, the signal processing operation becomes complex. Moreover, even when the edge of the second output signal OUT2 used for detecting the teeth on the rotor is switched, as shown in FIG. 1, the rotation-direction front edge of the rotor is detected when the rotor is rotating in the normal direction, and the rotation-direction back edge of the rotor is detected when the rotor is rotating in the reverse direction. In this way, different positions are detected depending on whether the rotor is rotating in the normal direction or the reverse direction. Therefore, it is difficult to accurately detect the rotational position of the rotor.